The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A190644 Least number k>1 such that (tau(k-1)+tau(k+1))/tau(k)=n where tau=A000005. 0
 6, 34, 39, 7, 11, 19, 29, 41, 79, 71, 179, 199, 181, 239, 883, 419, 701, 839, 881, 1429, 2351, 1259, 1871, 2161, 4049, 3079, 3361, 2521, 6481, 4159, 6299, 5279, 11551, 5039, 20789, 7561, 25919, 10079, 16561, 13441, 38611, 13859, 23761, 21839, 100673, 20161 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS MAPLE with(numtheory): a:= proc(n) local k;       for k from 2 while (tau(k-1)+tau(k+1)) /tau(k)<>n do od; k     end: seq(a(n), n=1..50);  # Alois P. Heinz, May 19 2011 MATHEMATICA tau = DivisorSigma[0, #]&; a[n_] := For[k=2, True, k++, If[(tau[k-1]+tau[k+1])/tau[k]==n, Return[k]]]; Array[a, 50] (* Jean-François Alcover, Mar 27 2017 *) PROG (Sage) def A190664(n):     tau = number_of_divisors     return next((k for k in IntegerRange(2, infinity) if tau(k-1)+tau(k+1) == n*tau(k))) # [D. S. McNeil, May 19 2011] CROSSREFS Cf. A000005 (number of divisors of n), A190612. Sequence in context: A183466 A112289 A283232 * A011798 A222174 A044464 Adjacent sequences:  A190641 A190642 A190643 * A190645 A190646 A190647 KEYWORD nonn AUTHOR Juri-Stepan Gerasimov, May 15 2011 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified September 18 01:39 EDT 2021. Contains 347504 sequences. (Running on oeis4.)